Re: [PATCH net] drivers: net: can: Fix deadlock in grcan_close()

[Oliver Hartkopp]

On 27.04.22 14:47, Andreas Larsson wrote:
> On 2022-04-26 21:12, Oliver Hartkopp wrote:
> > On 25.04.22 06:24, Duoming Zhou wrote:
> > > There are deadlocks caused by del_timer_sync(&priv->hang_timer)
> > > and del_timer_sync(&priv->rr_timer) in grcan_close(), one of
> > > the deadlocks are shown below:
> > >
> > >     (Thread 1)              |      (Thread 2)
> > >                             | grcan_reset_timer()
> > > grcan_close()              |  mod_timer()
> > >   spin_lock_irqsave() //(1) |  (wait a time)
> > >   ...                       | grcan_initiate_running_reset()
> > >   del_timer_sync()          |  spin_lock_irqsave() //(2)
> > >   (wait timer to stop)      |  ...
> > >
> > > We hold priv->lock in position (1) of thread 1 and use
> > > del_timer_sync() to wait timer to stop, but timer handler
> > > also need priv->lock in position (2) of thread 2.
> > > As a result, grcan_close() will block forever.
> > >
> > > This patch extracts del_timer_sync() from the protection of
> > > spin_lock_irqsave(), which could let timer handler to obtain
> > > the needed lock.
> > >
> > > Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
> > > ---
> > >   drivers/net/can/grcan.c | 2 ++
> > >   1 file changed, 2 insertions(+)
> > >
> > > diff --git a/drivers/net/can/grcan.c b/drivers/net/can/grcan.c
> > > index d0c5a7a60da..1189057b5d6 100644
> > > --- a/drivers/net/can/grcan.c
> > > +++ b/drivers/net/can/grcan.c
> > > @@ -1102,8 +1102,10 @@ static int grcan_close(struct net_device *dev)
> > >       priv->closing = true;
> > >       if (priv->need_txbug_workaround) {
> > > +        spin_unlock_irqrestore(&priv->lock, flags);
> > >           del_timer_sync(&priv->hang_timer);
> > >           del_timer_sync(&priv->rr_timer);
> > > +        spin_lock_irqsave(&priv->lock, flags);
> >
> > It looks weird to unlock and re-lock the operations like this. This 
> > breaks the intended locking for the closing process.
> >
> > Isn't there any possibility to e.g. move that entire if-section before 
> > the lock?
>
> All functions wishing to start the timers both check priv->closing and
> then, if false, start the timer within the priv->lock spinlock. Given
> that, it should be ok that del_timer_sync is not done within the
> spinlock as therefore no one can restart any timers after priv->closing
> has been set to true.
>
> It looks a bit weird, but setting priv->closing to true needs to happen
> within the priv->lock spinlock protection and needs to happen before
> del_timer_sync to avoid a race between grcan_close and someone starting
> the timer.
>
> Reviewed-by: Andreas Larsson <andreas@xxxxxxxxxxx>

Thanks Andreas!

Best regards,
Oliver